add named re2c:rules blocks

[skvadrik]

re2c should allow multiple re2c:rules blocks.
Different blocks should be addressed somehow, e.g. by name.

Original comment by: skvadrik

[skvadrik]

• labels: --> reuse

Original comment by: skvadrik

[skvadrik]

This also came up in #351.

Also, docs for reuse mode need to be updated as requested in: #351 (comment).

[skvadrik]

@hddharvey FYI I added support for named blocks and in-block use directive (#353). Here is a simple example: https://github.com/skvadrik/re2c/blob/master/examples/c/reuse/usedir.re. Docs are also updated, but not on re2c.org yet.